1. Field of the Invention
This invention pertains generally to destruction of contaminants in biogas, and more particularly to microwave induced destruction of siloxanes and hydrogen sulfide contaminants from biogas.
2. Description of Related Art
Developing biogas resources for electric generation is challenging because of pretreatment requirements to accommodate the generating equipment and combustion controls or post treatment systems needed to meet increasingly stringent emission requirements, especially in California. Removing siloxanes and hydrogen sulfide (H2S) from biogas is crucial because their combustion products increase engine maintenance intervals and interfere with existing post-combustion nitrogen oxides (NOx), sulfur oxides (SOx), and hydrocarbon removal technologies that are required to meet regional air quality standards.
Siloxanes are a family of man-made organic compounds that contain silicone, oxygen and methyl groups. As a consequence of their widespread use in consumer products, siloxanes are found in wastewater and in solid waste deposited in landfills. At wastewater treatment plants and landfills, low molecular weight siloxanes volatilize into digester and landfill gas. When this biogas is combusted in gas turbines, boilers, or internal combustion engines, siloxanes are converted to silicon dioxide (SiO2) and micro-crystalline quartz, which can deposit in the combustion and/or exhaust stages of the equipment as an abrasive white powder, contributing to engine wear and component failure. Landfills have siloxane concentrations on average from as low as 0.2 mg/m3 to about 10 mg/m3 but can be as high as about 140 mg/m3. Siloxane concentrations can vary greatly in landfills as they age whereas the concentration in wastewater digesters is fairly constant. Animal waste digesters typically contain little or no siloxane unless offsite waste material is added. Manufacturers of combustion turbines and reciprocating engines have begun limiting feed gas siloxane concentrations to between 5-28 mg/m3 for internal combustion (IC) engines, and 0.1 to 0.03 mg/m3 for gas turbines.
Table 1 presents common volatile siloxanes found in biogas with their molecular weight, vapor pressure, boiling point, chemical formula, and water solubility. Abbreviations are commonly used to identify the siloxane compounds. Siloxanes that are cyclic in structure have a single abbreviation of D. Siloxanes that have a linear structure have two abbreviations using an L or M nomenclature.
Additional organosilicon compounds such as Trimethylsilanol (Si(CH3)3OH) and Tetramethylsilane (Si(CH3)4) may also be present in biogas and are included in the term siloxane for purposes of describing this invention.
Both wet and dry scrubbers have been used to remove siloxanes from biogas. The major disadvantages of wet scrubbers are the production of hazardous liquid waste and the fact that complete silicon elimination is difficult to obtain since the highly volatile siloxanes are stripped from the solvent at elevated gas flow rates.
Cyclic and linear (dimethyl) siloxanes are very stable against chemical and biochemical degradation. However, strong acids or bases catalyze the cleavage of Si—O bonds to produce poly dimethyl siloxanes. Due to the high content of CO2 in biogases, the application of caustic absorbents for siloxane removal is not practical due to carbonate formation. Using acidic solutions is also difficult due to the hazardous liquid wastes.
The most common adsorbent used in dry scrubbing is Granular Activated Carbon (GAC), because it is cheaper than alternative adsorbents such as molecular sieves and polymer beads. GAC adsorbs siloxanes, H2S, heavy hydrocarbons and organic halides. Since siloxanes are difficult to desorb from GAC, the adsorbent must be replaced regularly. The siloxane saturated GAC is typically burned as fuel or disposed of in a landfill where the volatile compounds, including siloxanes, can reappear is subsequent landfill gas. Other contaminants, such as H2S, compete with siloxanes for adsorption sites on the GAC. Temperature and water content of the biogas also affects GAC adsorption capacity. Different siloxane compounds will also exhibit different adsorption capacities, generally with adsorption capacity increasing with increasing molecular weight.
Chilling biogas down to 40 deg F. is used to dry the gas for turbines and also removes siloxane in a range of 15% to about 50%. A proprietary process that refrigerates biogas to about −20 deg F. claims 95-99% siloxane removal rates. Both refrigeration processes use a GAC adsorber for final siloxane removal. Refrigeration uses significant energy to clean the biogas.
Consequently, GAC is currently the most practical and economic adsorbent for the removal of siloxanes in biogas. A better reactivation technology to desorb high molecular weight siloxanes can remove the need to chill the biogas and reduce the major lifecycle cost of GAC systems, which is carbon replacement. As discussed later, microwave energy can easily remove high molecular weight siloxanes from GAC.
Anaerobic digesters produce about 50%-60% methane, 40%-50% CO2 and sulfur impurities mostly in the form of H2S. Landfill gas typically has lower methane concentrations and added hydrocarbons derived primarily from solvents in the landfill. Landfill gas and digester gas may also contain a variety of trace compounds. More than 140 substances have been identified so far and they reach a total concentration of up to 2000 mg/m3 (0.15% by volume). During the combustion process, H2S and halogenated compounds in biogas form corrosive acids including H2SO4 and HCl.
Hydrogen Sulfide has a strong odor that can be detected at threshold levels of about 0.47 parts per billion (ppb) and has an OSHA IDLH level of 300 parts per million (ppm). Digesters have H2S levels of about 25 ppm to over 1,000 ppm for animal digesters, where landfills gas levels usually vary from 10 ppm to over 100 ppm. Assuming emissions of SOx are not an issue, boilers can tolerate H2S, levels up to 1,000 ppm, reciprocating engines about 10 to 100 ppm and fuel cells 10 ppm to 20 ppm.
Reciprocating engines operating on digester biogas compared to natural gas engines cost about 20% more to install and about 80% more to maintain. Sulfur plugs filters, causes deposits on valves and cylinders and contaminates lubricating oil. It has been reported that some operators must change spark plugs frequently ($1,000 annually) and change oil as often as weekly ($350 to $1,000 per month).
The H2S pretreatment system of choice for digesters with 100 to 1,000+ ppm H2S has been gas contact with an iron oxide media. The most well known treatment system is an iron sponge. This is a container of iron oxide impregnated media (typically woodchips) that scrubs the inlet gas from the digester. The iron sponge is sized for a residence time of about 60 seconds and, the media can collect up to about 2.5 times its weight in sulfur compounds. The media can be partially regenerated by exposure to air or by wetting for about 10 days. Eventually the media must be discarded and replaced with new media. With increasing frequency, the spent media is classified a hazardous waste by local regulators. One example of an iron sponge system costs about $50,000 to install with annual operating costs ranging from $250 to $4,000.
Proprietary iron-oxide media such as SulfaTreat®, Sulfur-Rite®, and Media-G2® have been installed as improved alternatives to the iron sponge at a few digester sites. These use different media and additional chemical treatment to remove sulfur. Some of these media have limited regeneration capacity or can be safely deposited in a landfill. One dairy digester site using Media-G2 has two vessels with about 760 kg of media each with a residence time of about 62 seconds per vessel. Annual media consumption ranges from 1,460 kg to about 5,900 kg with media replacement costs on the order of $2,050 to $8,290.
GAC and other carbon products are used extensively for filtration of contaminants in water and gas streams. GAC contains micro-pores that capture and hold many organic and polar molecules and is more effective for larger molecules. In other cases, the carbon acts as a catalyst to drive a reaction with the carbon and the selected molecule in a process known as chemisorption.
Commercially available GAC and Pelletized Activation Carbon (PAC) have the surface area in the range of 800-1000 m2/g. These activated carbons easily adsorb SO2, NOx, and VOCs. The carbon adsorption capacity is dependent on the composition of gas. GAC and PAC also adsorb siloxanes and H2S in biogas.
GAC adsorbs most VOCs and is used in removing common solvent vapors used in drying cleaning and parts washing operations. The carbon adsorption capacity is strongly dependent on the VOC molecular weight. The adsorption capacities of toluene and methylene chloride at the room temperature are 20 and 5 g/100 gGAC, respectively. However the adsorption capacity of CH4 in GAC is negligible.
The GAC adsorption capacity for H2S is 5-15% by weight depending on loading of water and other contaminants. Therefore, GAC can be used economically to remove the H2S from biogas that contains lower concentrations of H2S. Typically, used GAC is disposed of in a landfill when saturated with H2S.
Impregnating GAC with alkaline or oxide solids enhance the physical adsorptive characteristics of the carbon with chemical reaction. Sodium hydroxide (NaOH), sodium carbonate (Na2CO3), and potassium hydroxide (KOH) are common impregnators. The metal oxide impregnation increases the GAC adsorption capacity significantly-especially if a small amount of oxygen is present in the biogas stream. Typically, 20-25% loading by weight of H2S can be achieved. The metal-impregnated GAC is almost twice more expensive than GAC. However, the use of metal-impregnated GAC will be more economical for the adsorbers without the on-site carbon reactivation because of its greater adsorption capacity. If the on-site GAC regeneration is available, the use of regular GAC for H2S removal is preferable to the metal oxide-impregnated GAC.
The GAC adsorption capacity for siloxanes has been reported to be from 1 to 1.5 percent by weight. This capacity is affected by the species of siloxane in the gas, other contaminants in the gas including H2S, and the temperature and water content of the gas.
Once GAC can no longer adsorb a chemical compound, breakthrough will occur where the compound will flow all the way through the bed without being adsorbed. At this point, the GAC is no longer effective and must be replaced. In many cases, such as GAC filled water filters or respirators, the GAC is thrown away and a fresh GAC filter or cartridge is installed. In large scale processes, or where the contaminant can be recovered or destroyed, regeneration of the GAC may be preferred.
There are four processes commonly used for GAC regeneration: Temperature Swing Adsorption (TSA), Pressure Swing Adsorption (PSA), Inert Purge, and Displacement Purge. TSA takes place by heating the GAC to remove contaminants. With PSA the adsorption takes place at an elevated pressure and regeneration at a lower pressure. Inert gas purge reduces the partial pressure of the adsorbate in the gas phase so that desorption occurs. A purge gas that is more strongly adsorbed than the contaminant is used to desorb the original contaminant. Steam regeneration is a combination of TSA and purge. In each process, the contaminant is still present in the purge stream and must be captured, burned or vented to the atmosphere.
High molecular weight siloxane compounds make conventional thermal regeneration difficult due to their low vapor pressures. Heavy molecules accumulate in GAC after thermal regeneration, limiting the number of activation cycles carbon can be subject to before its performance is inadequate.
A molecular sieve is a material containing tiny pores of a precise and uniform size that is used as an absorbent for gases and liquids. Molecules small enough to pass through the pores are absorbed while larger molecules are not. It is different from a common filter in that it operates on a molecular level. For instance, a water molecule may be small enough to pass through while larger molecules are not. Because of this, they often function as a desiccant. A molecular sieve can absorb water up to 22% of its own weight so removal of water in biogas is important. Often they consist of aluminosilicate minerals, clays, porous glasses, microporous charcoals, zeolites, active carbons, silica gel or synthetic compounds that have open structures through which small molecules, such as nitrogen and water can diffuse. They are classified by pore size such as type 3A or type 4A designating pore size in angstroms. Traditional methods for regeneration of molecular sieves include pressure change, as in oxygen concentrators, or by heating and purging with a carrier gas. Molecular sieve materials can also be regenerated in a microwave system.
Quantum radiofrequency (RF) physics is based upon the phenomenon of resonant interaction with matter of electromagnetic radiation in the microwave and RF regions since every atom or molecule can absorb, and thus radiate, electromagnetic waves of various wavelengths. The rotational and vibrational frequencies of the electrons represent the most important frequency range. The electromagnetic frequency spectrum is usually divided into ultrasonic, microwave, and optical regions. The microwave region is from 300 megahertz (MHz) to 300 gigahertz (GHz) and encompasses frequencies used for much communication equipment. For instance, refer to Cook, Microwave Principles and Systems, Prentice-Hall, 1986.
Often the term microwaves or microwave energy is applied to a broad range of radiofrequency energies particularly with respect to the common heating frequencies, 915 MHz and 2450 MHz. The former is often employed in industrial heating applications while the latter is the frequency of the common household microwave oven and therefore represents a good frequency to excite water molecules. In this writing the term “microwave” or “microwaves” is generally employed to represent “radiofrequency energies selected from the range of about 500 to 5000 MHz”, since in a practical sense this large range is employable for the subject invention.
The absorption of microwaves by the energy bands, particularly the vibrational energy levels, of atoms or molecules results in the thermal activation of the nonplasma material and the excitation of valence electrons. The nonplasma nature of these interactions is important for a separate and distinct form of heating employs plasma formed by arc conditions at a high temperature, often more than 3000 degrees F., and at much reduced pressures or vacuum conditions. For instance, refer to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Supplementary Volume, pages 599-608, Plasma Technology. In microwave technology, as applied in the subject invention, neither of these conditions is present and therefore no plasmas are formed.
Microwaves lower the effective activation energy required for desirable chemical reactions since they can act locally on a microscopic scale by exciting electrons of a group of specific atoms in contrast to normal global heating which raises the bulk temperature. Further, this microscopic interaction is favored by polar molecules whose electrons become easily locally excited leading to high chemical activity; however, nonpolar molecules adjacent to such polar molecules are also affected but at a reduced extent. An example is the heating of polar water molecules in a common household microwave oven where the container is of nonpolar material, that is, microwave-passing, and stays relatively cool.
In this sense, microwaves are often referred to as a form of catalysis when applied to chemical reaction rates; thus, in this writing the term “microwave catalysis” refers to “the absorption of microwave energy by carbonaceous materials when a simultaneous chemical reaction is occurring” For instance, refer to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Volume 15, pages 494-517, Microwave Technology.